Heating and cooling systems for structures housing industrial and business functions now often contain internally disposed rooms or confined spaces wherein there is provided a very precisely controlled environment such to assure the proper performance of complex and highly sensitive equipment such as computers. A specialized environmental control generally will include refrigeration and/or air conditioning components as well as humidity regulation devices combined with sensing systems which are called upon to operate continuously on a year-round basis. Generally, the refrigeration or air conditioning equipment utilized for this function is dedicated and involves the operation of compressor components in conjunction with evaporators, expansion devices and condensers. The latter, condensing function generally is one which is located to carry out a heat exchange operation with the air of an outdoor, ambient surrounding. Inasmuch as a significant number of these dedicated installations are located in climates having seasons of varying winter and summer temperatures, approaches have been made to take advantage of cooler temperatures to lower energy expenditures otherwise required for the systems. For example, in one approach, advantage has been taken of seasonally lower outdoor or ambient temperatures by a controlled mixing of cooler outdoor air with recirculated air within the confined space. This commingling of air permits an intermittent deactivation of energy consumming refrigeration components of the environmental control systems.
One disadvantage attendant with the use of outside air to contribute to cooling stems from the general tendency of industry to locate computer rooms and the like well within the internal regions of buildings as noted above. As a consequence, access to outside air can be achieved only through the use of extensive and bulksome ducting, a requirement rendering the otherwise achieved energy conservation impractical and economically unsound. However, should the location disadvantage be overcome, the typically encountered variations in ambient air humidity have been found to counteract the energy gains from utilizing outdoor air and lower temperature, inasmuch as a greater consumption of energy is required to carry out humidification and dehumidification than initially conserved in using the outside air. This outside air also will contain undesirable contaminants such as dust and the like.
The first practical system introduced to industry where an effective energy conservation was achieved taking advantage of lower outside or ambient air temperatures is described in U.S. Pat. No. 3,525,385 under the inventorship of Ralph C. Liebert. The system provides an outdoor heat exchanger which operates in conjunction with a liquid heat exchanging medium, such as glycol or the like. This glycol is pumped through conduits into the building retaining the enclosed air conditioned region and at which point the fluid is utilized both to provide heat exchange within the condenser units of the refrigeration system and, alternately, to provide cooling through the use of a liquid cooling coil interposed within the air flow of the internally disposed air conditioning equipment. Thus, as outdoor temperatures drop below predetermined levels, i.e. about 35.degree. F., full advantage is taken of those lower levels to accommodate the heat loads imposed from computer equipment and the like.
U.S. Pat. No. 4,271,678 under the inventorship of Ralph C. Liebert describes an improvement to the above noted system wherein advantage is taken of the cooling capabilities of outdoor air at temperatures between 35.degree. F. and 65.degree. F. The patent looks to the average temperature data available for most American cities and notes that in geographical areas currently comprising about 75% of industry and commerce, outdoor temperatures within that range occur for at least one-half of a given year.
In a copending application for United States patent entitled "Energy Efficient Air Conditioning System Utilizing a Variable Speed Compressor and Integrally Related Expansion Valves" by Sillato and Baer, Ser. No. 565,407, filed Dec. 27, 1983, and assigned in common herewith, an extremely efficient air conditioning and/or refrigeration system is described which shows a combination of a variable speed compressor with integrally related expansion components which perform for the instant purpose of controlling the environment of computer rooms and the like in a manner achieving efficiencies not heretofore realized. With that system, the stepped, on and off operation of the compressor is substantially reduced to, in turn, minimize strain otherwise asserted upon the equipment. To achieve a requisite performance of this system, a controller generally referred to as an inverter drive is required which is capable of reliable long-term performance and which enjoys a maximum of efficiency. In the latter regard, the inverter is called upon to assert drive to an inductive compressor motor providing for optimum performance at speeds as low as 20 percent of nominal without the development of phenomena such as motor slip or the like. Further, the inverter must be fabricable for efficient operation in highly compact form so as to be mountable with the air conditioning equipment which it drives without imposing any significant change in cabinetry size.
Because the environments of computer rooms and the like, for the most part, are of themselves monitored by microprocessor controlled components, the inverter drives now contemplated must also be controllable from the firmware or software programs of the processors. Further, the inverter drives or controllers must exhibit a very high reliability, working off a conventional three-phase 480 volt a.c. input line. For example, the inverter drives should be capable of accommodating short term single phase conditions and must be immune to any of a wide variety of transients which are encountered either from line sources or from coupled motor defects. Additionally, to achieve acceptable cost factors for the entire systems, the controller or inverter structures must be fabricable at acceptable price levels, for example, incorporating designs which advantageously lessen ampacity requirements. In the latter regard, the controllers must be capable of accommodating for start-up phenomena which generally involves encounters with very high level surge currents.